1. Field of the Invention
The present invention relates to a sheet post-processing apparautus adjacent to a sheet discharging section of an image forming system such as a copier, a printer, a facsimile and so on. More particularly, it relates to a sheet post-processing apparatus capable of performing post-process such as a stapling or a punching to sheets on which images are formed by the image forming system.
2. Description of Related Art
Recently, the image forming system is developed from analogue type into digital type. The digital image forming appartus can perform one or more function(s) of copy function, print function, facsimile function and selectively discharge the sheets on which images are formed.
That is, since the digital image forming system can store image data to be copied or printed, the image forming system can print the images on the sheets and discharge the sheets on which the images are formed, according to sequency which user wants. Therefore, a sheet post-processing apparatus of the analogue image forming system requires a plurality of accommodation trays for sorting and accommodating the sheets, but a sheet post-processing apparatus of the digital image forming system, without a plurality of accommodation trays, can sort and accommodate the sheets by only one accommodation tray for temporarily accommodating the sheets until sheets are completely discharged from the image forming system.
As an embodiment of the new sheet post-processing apparatus, when the digital image forming system copies a manuscript consisting of a plurality of sheets to make a plurality of duplicates, the image forming system repeatedly performs operation which prints and discharges the manuscript successively until all the sheets are accommodated into the accommodation tray. Thereafter, the sheet post-processing apparatus accommodates the sheets into a tray which has a stacker function. In this case, if necessary, the sheet post-processing apparatus can staples(or punches) the discharged sheets before accommodating the sheets into the stacker.
Further, the accommodation tray for temporarily accommodating the sheets has an aligning function to align the sheets accommodated therein and an offset function to accommodate bundles of sheets to be post-processed in zigzags.
The sheet post-processing apparatus of the digital image forming system is disclosed in U.S. Pat. No. 5,385,340 and U.S. Pat. No. 5,021,837, and the sheet post-processing apparatuses are briefly explained in the following, referring to FIGS. 1 and 2.
As shown in FIG. 1, the conventional sheet post-processing apparatus is adjacent to a discharging section 2 of the image forming system 1 and is provided with a copy sheets discharged from the discharging section 2. The copy sheets transferred to the conventional sheet post-processing apparatus are discharged from a sheet oulet 9 by a conveyer 4 and is accommodated into an aligning tray 5 slantedly disposed. In this case, an end of each of the copy sheets is supported by a pusher 8 disposed at lower portion of the aligning tray 5 so that the copy sheets may be aligned. Further, in order to accurately align the copy sheets, the conventional sheet post-processing apparatus has a roller 7 for moving the copy sheets accommodated into the aligning tray 5 toward pusher 8.
When the process as above-mentioned is repeated and the sheets are completely discharged from the image forming system, a solenoid 5a connected to the aligning tray 5 operates to align a side of each of the sheets accommodated into the aligning tray 5. Then, the conventional sheet post-processing apparatus performs a post-process by a punch(not shown) or a stapler 10 to the aligned sheets. Thereafter, the pusher 8 supporting the end of each of the sheets is forwardly moved by a motor 8a so that the post-processed sheets are accommodated into an accommodation tray 6.
Further, the conventional sheet post-processing apparatus has a motor 6a capable of vertically moving the accommodation tray 6. The motor 6a moves the accommodation tray 6 to the position suitable for accommodation of the sheets. That is, when a bundle of sheets is accommodated into the accommodation tray 6, the motor 6a moves the accommodation tray 6 downward by a predetermined distance so that upper end of the accommodation tray 6 is positioned in place capable of accommodating the next bundle of sheets.
Furthermore, the conventional sheet post-processing apparatus has a motor 6b for horizontally moving the accommodation tray 6 in order to offset the sheets. As shown in FIG. 2, when a bundle of sheets is completely accommodated into the accommodation tray 6, the motor 6b moves the accommodation tray 6 horizontally. At this time, when the next bundle of sheets is accommodated into the accommodation tray 6, the previous bundle of sheets and the next bundle of sheets are positioned in zigzags. Therefore, when a plurality of bundles of sheets are accommodated into the accommodation tray 6, it may be prevented that staples imbeded into the bundles are stacked and a portion of the sheets accommodated into the accommodation tray is higher than the other portions.
However, since the conventional sheet post-processing apparatus aligns a side of each of the sheets accommodated into the aligning tray by the solenoid, there is a problem that sizes of the sheets used for the apparatus should be uniform.
Further, when a plurality of sheets are accommodated into the accommodation tray, since a large of load is applied to the motor for horizontally moving the accommodation tray, expensive motor capable of generating great driving force should be used for the apparatus in order to bear the load.
Furthermore, since the conventional apparatus has a plurality of elements for moving the sheets(that is, a pusher, a motor to operate the pusher and so on), the structure is complex.